1. Field of the Invention
This invention relates to a DC circuit breaker which uses the blowing of an arc-extinguishing gas onto the arc.
2. Description of the Prior Art
In the case of AC there is a time point during the passage of the current in which the current is instantaneously zero, but in the case of DC there is no such time point. Circuit breaking of DC is therefore not as easy as in the case of AC. However, circuit breaking of DC can be made easier by superimposing AC on the DC, so that the current does have a condition in which it is instantaneously zero, i.e. a current zero point. One example of a development which has been made with this object in view is the connection, in parallel with the pair of electrodes of the circuit breaker, of a series L-C circuit consisting of a coil and a capacitor. The arc onto which the arc-extinguishing gas is blown has a marked negative resistance characteristic so that strong oscillations are generated in the oscillation circuit represented by the series L-C circuit and a progressively increasing oscillation current flows in this circuit. In this condition the arc current is the result of superposition of the DC current which is to be interrupted and the oscillating current. The instantaneous current thus varies in an oscillating manner, with gradually increasing amplitude, about the value of the direct current mentioned above. Ultimately, a time point is reached at which the current flowing between the two electrodes is zero. The DC current passing through the circuit breaker is interrupted at this point.
Although the above method of circuit breaking is effective, the development of a DC circuit breaker is required that can interrupt large currents reliably in a short time and which is of smaller size.